The invention relates to a valve arrangement comprising a housing, a shut-off element rotatably disposed in the housing, and a shaft which is coupled to the shut-off element for rotation therewith and which extends out of the housing and is sealed by means of a sealing element.
U.S. Pat. No. 4,930,748 discloses such a valve arrangement configured as a shut-off and control valve. A shut-off element, which is configured as a plug, is affixed to a shaft and disposed for rotation in a housing, two seals being provided to seal off the shaft at the top cap of the housing. The first seal is supported on an annular disk on the bottom of the cap, inside of the housing, while the second seal is associated with the exterior and if necessary it can be adjusted from the outside. Since the shut-off element is configured as a plug the axial position of the shaft can vary due to production tolerances, wear, external forces or the like. Since the inner, first seal is supported on the annular disk a securely operating seal is achieved independently of the axial position of the stem. The increased safety prescriptions and standards for environmental protection are satisfied, and the required limits of leakage rates are maintained for a long useful life. This valve arrangement is designed for special applications and uses and its production involves considerable manufacturing cost and complexity.
Pipe fittings lined or coated with plastic are used primarily in the chemical industry, especially fittings lined or coated with polytetrafluoroethylene (PTFE), fluoroethylene propylene (FEP) or perfluoroalkoxy (PFA). The manufacturing cost and expense, especially for the tools needed for producing plastic linings for valve housings and stems, are considerable. If the safety requirements imposed upon existing plants are increased, then the fittings must regularly be replaced with new fittings which satisfy the higher requirements. The costs of maintaining stocks of valve fittings in such cases are considerable for both manufacturers and dealers.
German Utility Model DE-U 89 10 895 discloses a valve arrangement which is configured as a ball valve in which the housing and also the ball are provided with a plastic coating. A shaft is formed integrally on the ball disposed for rotation in the housing, passes through the housing, and is surrounded by a special packing box. Disposed in the packing box is a sealing element against which a spring thrusts. This added-on packing box is arranged partially outside of the housing and another part extends into the interior of the housing as far as the ball. During manufacture, extra measures must be taken to assure dependable sealing of the packing box with respect to the valve housing. Also, the packing box is provided with a plastic coating which necessitates considerable manufacturing cost. Lastly, the packing box contains outwardly leading openings in the vicinity of the above-mentioned spring, and if the sealing element is damaged the medium flowing through the valve arrangement can escape through these openings.
German Utility Model DE-U 84 31 238 discloses a re-linable double packing box arrangement for a ball valve. An extension piece with a bottom plate, a comparatively long tube and a top plate are provided, and a spindle extension is disposed in the long tube. The tube and the two plates are joined together by weld seams. Furthermore, this arrangement has no coating or lining and is not readily suitable for use under high safety conditions.
German Patent No. DE 40 16 541 discloses a packing box packing which has active and passive sealing rings in a receiver. The passive packing box rings are designed for the transmission of axial forces and lie alternately with their radial inside surface upon the shaft or with their radial outside surface upon the wall of the packing box seal. A lining or coating is not provided.